The main goal of this proposed research this proposed research is to determine how the timing of surface antigen switching is controlled in C. elegans. The investigator has isolated mutations in several genes that affect the antigenic properties of the surface of the nematode. Srf-2 and srf-3 (srf-d) mutants fail to produce an antigen, detected by M37 and M38 antibodies, that is specific to the L1 surface of wildtype worms proceeding through continuous development. By immunostaining, these mutants also appear to have altered surface antigens throughout development that may result from an unmasking of normally hidden antigens. In srf-6 mutants the M38 antigen is present on the surface of not only L1 larvae but also all other larval stages. Thus, srf-6 mutants fail to switch their surface antigen at the end of the L1 stage. This phenotype has been called the "Srf heterochronic phenotype:. The investigator has also shown that the Srf heterochronic phenotype is exhibited by dauer-constitutive mutants (daf-c) in six out of seven genes that have been tested); this phenotype is suppressed in daf-c mutants by dauer-defective (daf-d) mutants in two of three genes tested that lie downstream in the dauer regulatory pathway. These results implicate the genes that regulate decisions between continuous development and entry into the dauer pathway of development as also regulatory surface antigen switching. The investigator proposes to study the role of srf-6 in surface antigen switching molecularly by determining th physical location and sequence of the srf-6 gene. The deduced amino acid sequence of the Srf-6 polypeptide will be compared to sequences in databanks and may provide clues as to srf- 6 function. Whether srf-6 is regulated transcriptionally during normal development and in daf-c mutant backgrounds will be examined by northern blots. The timing and anatomical location of srf-6 expression will be analyzed in transgenic animals containing a lacZ reporter fused to srf-6. These expression studies may also provide information concerning srf-6 function. The interaction of srf-6 with other genes will be investigated genetically under the second specific aim. Existing mutants in daf-c genes will be tested for genetic interactions with srf-6 by looking for enhancement or alteration of phenotype in double mutants. These experiments may indicate whether the daf-c genes and srf-6 lie in the same or independent pathways to control surface antigen switching. Additionally, the control of surface antigen switching will be studied by seeking new mutants that display a phenotype similar to srf-6. These mutants will be identified as expressing the M37 or M38 antigen as L4 larvae. Mutants will be isolated in a daf-3 (daf-d) background to avoid isolating daf-c mutants. Preliminary data indicate that srf-6(yj43) interacts genetically with srf-2(yj262) so that the srf-d phenotype of srf-2 is partially suppressed. This interaction will be examined further using more extensive sets of available srf-2 and srf-6 alleles to ask whether the interaction is allele-specific and whether the srf -2 and srf-6 products may interact directly. New mutations that suppress the srf-2 phenotype will also be sought to identify additional genes or additional alleles of srf-6 that interact with srf-2. Finally, the chemical stimuli hat are responsible for the Srf heterochronic phenotype will be tested by assaying whether the dauer pheromone and/or other components of culture medium elicit the response and whether different classes of chemosensory mutants can respond to spent culture media by failing to switch off the Li-specific surface antigen.